The photocatalytic properties of titanium dioxide result from the promotion of electrons from the valence band to the conduction band under the influence of ultraviolet (UV) and near-UV radiation. The reactive electron-hole pairs that are created migrate to the surface of the titanium dioxide particles where the holes oxidize adsorbed water to produce reactive hydroxyl radicals and the electrons reduce adsorbed oxygen to produce superoxide radicals, both of which can degrade NOx and volatile organic compounds (VOCs) in the air. In view of these properties, photocatalytic titanium dioxide has been employed in coatings and the like to remove pollutants from the air. Such coatings may also have the advantage of being self-cleaning since soil (grease, mildew, mold, algae, etc.) is also oxidized on the surface.
Coatings containing photocatalytic titanium dioxide can be made using many different types of binders or resins systems. Typically, photocatalytic coatings are prepared with polysiloxane binders because of the excellent stability of polysiloxane polymers to oxidative conditions. Binders and resins systems composed of carbon, hydrogen, and oxygen rapidly undergo photooxidation to produce water, carbon dioxide and, if nitrogen is present, nitrogen-containing species in the presence of UV light.
Although coating compositions comprising siloxane type polymers show excellent durability, the cost of siloxane type polymers is significantly higher than the cost of other organic polymers such as acrylic or styrene polymers. Using mixtures of siloxane polymers with organic polymers in coating compositions improves the durability of the corresponding coatings over organic polymer based compositions alone, but still results in progressively diminished durability as the siloxane polymer concentration is reduced. Accordingly, it has also been difficult to provide coatings having high levels of photocatalyst because the catalyst tends to oxidize and break down the polymeric binder of the coating. This problem is exacerbated when the coating is exposed to intense UV radiation from direct sunlight, as is the case with an exterior paint.
Topcoat coatings based on epoxy siloxane polymers are well known and provide coatings with the excellent corrosion resistance of an epoxy coating with the weatherability of a polyurethane in one coating. Since the epoxy siloxane polymer is produced from the reaction of an epoxy resin and a cross-linking component, they are typically a two component system that must be combined prior to application of the coating. These types of coatings find wide use in marine and industrial settings where durability and resistance to corrosion is critical. For example, epoxy siloxane based coatings are used on structural steel, tank exteriors, and piping and the like. Furthermore, epoxy siloxane coatings do not contain isocyanate residues, which present a safety hazard to personnel applying the coating. Use of epoxy siloxanes in combination with inorganic zinc silicate primers allow the use of a single coat of an epoxy siloxane coating rather than one coat of an epoxy-based coating and another coat of a urethane coating. The ability to provide a high quality coating in one coat, presents a significant labor cost savings and minimizes down time of equipment.
It is therefore an object of the present invention to provide durable photocatalytic coatings on substrates and coating compositions for applying said coatings. In particular, paint coatings and coating compositions, which comprise titanium dioxide photocatalysts capable of removing pollutants from the air and self-cleaning are provided. It is a particular object of the invention to provide durable coatings comprising photocatalytic titanium dioxide and an epoxy siloxane binder capable of removing pollutants from the air and self-cleaning.
International Application Publications Nos. WO2005/083014, WO 2006/030250, and WO 2005/083013 to Goodwin et al., all of which are hereby incorporated by reference in their entirety, describe self-cleaning and de-polluting coating compositions comprising photocatalytic TiO2.
U.S. Pat. No. 4,250,074 to Foscante et al., which is hereby incorporated by reference in its entirety, describes coatings that comprise an interpenetrating polymer network of a polymerized epoxy resin intertwined with a polysiloxane polymer. The epoxy resin is cured with an amine curing agent or with an aminosilane.
U.S. Pat. Nos. 5,618,860 and 5,804,616 to Mowrer et al., which are hereby incorporated by reference in their entirety, describe an epoxysiloxane based coating composition. The composition is prepared by combining a blend of an aromatic epoxy resin with a polysiloxane and an organooxysilane, an amine or aminosilane hardener, a catalyst and an aggregate or pigment component.
U.S. Pat. No. 7,026,398 to Monkiewicz et al., which is hereby incorporated by reference in its entirety, describes an air-drying coating composition that comprises at least one epoxy resin, at least one acrylic resin, at least one acrylic monomer, at least one copolymer based on an acrylic monomer or epoxy monomer, optionally a silicic ester and or alkyl silicate, at least one aminoalkylsilane and optionally an organosilane or mixture of organosilanes.
U.S. Pat. No. 7,160,962, which is hereby incorporated by reference in its entirety, to Sakugawa describes elastomer-modified epoxy polysiloxane compositions. The compositions are prepared by combining a silicone intermediate with an epoxy resin, an elastomeric resinous intermediate, a polyfunctional amine and optionally a catalyst, pigments and other processing agents.
The foregoing discussion is presented solely to provide a better understanding of the nature of the problems confronting the art and should not be construed in any way as an admission as to prior art nor should the citation of any reference herein be construed as an admission that such reference constitutes “prior art” to the instant application.